Measurement of power output of a nuclear fuel assembly is a critical task undertaken periodically at nuclear power plants throughout the world. Power output of fuel assemblies is measured on a spot and continual basis to ensure that the nuclear reactor core is behaving as expected and designed with various methods, usually in-core detectors monitoring specific characteristics of nuclear radiation. Furthermore, nuclear power operators are specifically required to operate the nuclear reactor within certain performance limits to guarantee reactor and component safety. If unexpected transients occur and regions of the nuclear reactor core are undergoing unanticipated levels of nuclear activity, the fuel assemblies (rods) in that specific region may become depleted too quickly rendering that area of the nuclear core less viable for continued operation. As a result of the depleted fuel assemblies, the fuel assemblies in this region will have to undergo remedial measures to minimize the unexpected depletion. The depleted fuel assemblies may be replaced with fresh fuel assemblies and the core reshuffled (i.e. the fuel assemblies moved to different positions in the core). In other instances, local power of fuel assemblies may be less than designed due to the depleted assemblies, thereby requiring control rods to be withdrawn or, alternatively, chemical shims removed from the coolant to increase reactivity in the core. Operating a nuclear reactor in an underpowered or overpowered mode negatively impacts the economics of the facility.
The locally overpowered fuel rod has small portions where the power evacuated exceeds the average power of the rod. If the deposited CRUD has the maturity conditions, (higher density for a given composition), the local temperature of the fuel rod, in such locations, can exceed the safe operating limits, leading to fuel failure.
This also negatively impacts the economics of the operation of the power plant.
Analysis of the deposit in such locations, usually also uncovers small regions where the CRUD deposition has reconfigured itself to evacuate more power than the design limits. This reconfiguration is identifiable during warranty post failure CRUD analysis. Facility operators, therefore, strive to identify underpowered core situations and quickly remedy these conditions.
In order to prevent infringement of operating safety and performance margins operators initiate safeguards, both physical and procedural, to ensure safe operation of the plant. Among the physical safeguards implemented, for example in a boiling water reactor, local power monitors are placed within the core to measure the amount of power being generated at specified positions in the core. These measurements provide operators with a snapshot of the core at these measurement locations. In-core monitors, however, are not placed in all locations of the core as it is impractical to install measuring equipment in all locations of a reactor core. Areas of the core, consequently, go unmonitored without sacrificing safety of operation. After core operation, assumptions are then made as to the amount of useful life remaining for each of these non-measured assemblies. Placement of these non-measured assemblies back into the core involves conservative assumptions for the remaining life of the assembly. Because conservative assumptions are made, nuclear fuel may be discharged from the core as supposedly “depleted”, when, in fact, there is sufficient fuel left in the fuel assembly for further operation. Operating the core in an inefficient manner can negatively impact the economic aspects of the nuclear facility.
Fuel assemblies change nuclear reactivity during core exposure time, thereby complicating the identification of the remaining life of each fuel assembly. Unidentified materials, known as CRUD, can coat or be deposited on the outside of fuel rods and assemblies. All are affecting the heat transfer capability of the reactor components. Deposits can also form on other heat transfer surfaces, such as steam generator tubing. As the deposit layer thickness increases, an insulating effect occurs for the nuclear component, for example, hindering heat transfer and power output of the core.
CRUD can significantly affect the remaining life of each fuel assembly in the core. In reactor operation, however, CRUD deposits differ at each location in the reactor. The differing amounts/thicknesses of CRUD deposits, therefore, hinder reactor engineers in determining the amount of useful life left in a nuclear fuel assembly because some fuel assemblies have a significant amount of insulating CRUD while other fuel assemblies do not.
There is therefore a need to provide a method to determine the power transfer characteristics of a nuclear fuel assembly which has accumulated CRUD deposits on nuclear fuel rods.
There is furthermore a need to provide a method to determine the power transfer characteristics of fuel assemblies that have a core residence time, but however were not physically monitored during core exposure.
There is also a need to provide a method that will determine the power transfer characteristics of core components, such as steam generators, that have an accumulated deposits on their tubing surface.